1. Field of the Invention
The present invention relates to a microphone unit which detects sound (i.e. vibration of air) and converts the detected sound to an electrical signal as an output signal.
2. Description of the Related Art
A microphone unit having a vibratory diaphragm for detecting sound is known in which sound is guided to front and rear surfaces of the vibratory diaphragm so as to detect the sound by the vibration of the vibratory diaphragm due to a difference between sound pressures on the front and rear surfaces of the vibratory diaphragm (i.e. difference in phase and amplitude between sound waves or vibrating waves of air incident on the front and rear surfaces of the vibratory diaphragm). It is a kind of differential type microphone unit, and has a figure “8” shaped bi-directional characteristics (pattern). Such a microphone unit has an effect to reduce far-field noise (reduce detection sensitivity to detect sound emitted from a far position) as compared with a non-directional (omni-directional) microphone unit which guides sound to only one of the front and rear surfaces of the vibratory diaphragm.
FIG. 10 is a graph showing relationship between sound source distance (position from which the sound is emitted) and detection sensitivity in a differential type microphone unit and a non-directional microphone unit. As apparent from the relationship shown in FIG. 10, the difference between the detection sensitivity to sound emitted from a near position and that emitted from a far position (reduction degree of detection sensitivity to sound emitted from a far position relative to that emitted from a near position) is larger in the case of the differential type microphone than in the case of the non-directional microphone. It can be understood from this that the differential type microphone unit has an effect to reduce far-field noise as compared with the non-directional microphone unit.
Generally, such a differential type microphone unit has a housing and a vibratory diaphragm placed in the housing. The housing has a first opening and a second opening which allow sound to be input therethrough such that the sound input through the first opening is guided to the front surface of the vibratory diaphragm, while the sound input through the second opening is guided to the rear surface of the diaphragm. Now considering positions from which sound is emitted (positions of the sound source) in the conventional differential type microphone unit, there exits a position where the phase of the sound incident on the front surface of the vibratory diaphragm is equal to that on the rear surface of the vibratory diaphragm. Such a position is referred to as a null point.
In the conventional differential type microphone unit, assume a structure that the sound propagation time from the first opening to the front surface of the vibratory diaphragm is equal to that from the second opening to the rear surface of the vibratory diaphragm. Under this assumption, the null point is formed at a position where the sound propagation time from the sound source to the first opening is equal to that to the second opening, namely at a position where the distance from the sound source to the first opening is equal to that to the second opening. Thus, in the conventional differential type microphone unit with the assumed structure, sound emitted from the null point causes a sound wave input through the first opening to be equal to that through the second opening both in phase and amplitude, making the sound wave incident on the front surface of the vibratory diaphragm equal to that incident on the rear surface of the vibratory diaphragm both in phase and amplitude. Thus, in the conventional differential type microphone unit with the assumed structure, the sound emitted from the null point causes the sound pressures on the front and rear surfaces of the vibratory diaphragm to be completely cancelled by each other, preventing the vibration of the vibratory diaphragm and resulting in a zero detection output for the sound emitted from the null point.
When mounted in a product such as a mobile phone, the conventional differential type microphone has an advantage that it can receive a voice of a close talker (user) and reduce far-field noise. However, there is a problem that if the mouth of the talker (user) is positioned at a null point, the voice (sound) of the talker is significantly reduced in level, making it impossible to recognize the talking voice. This is particularly so in a mobile phone 90 shown in FIG. 11 which is a schematic front view showing an example of mounting a conventional differential type microphone unit 80 in the mobile phone 90. Referring to FIG. 11, the mobile phone 90 has sound receiving openings 92a, 92b formed on one side thereof, while the differential type microphone unit 80 has first and second openings 81a, 81b which face the sound receiving openings 92a, 92b, respectively, and are placed on the same side on which the sound receiving openings 92a, 92b are placed. Such an arrangement is likely to cause a problem described above, preventing good voice quality.
There are other known microphone units in the art. For example, Japanese Laid-open Patent Publication 2007-180896 discloses a sound (audio) signal processing device with a bi-directional (first) microphone and a non-directional (second) microphone placed close to each other, in which output signals of the first and second microphones are processed to extract therefrom a signal having a predetermined correlation so as to allow the directional characteristics to be high in a narrow angular range. Japanese Patent 3620133 discloses a stereo microphone having four microphone capsules, in which output signals of the four microphone capsules are processed to obtain a stereo sound (audio) signal.
Japanese Laid-open Patent Publication 2003-44087 discloses an ambient noise reduction system with multiple microphones, in which input signals of the microphones are processed to subtract therefrom sound (audio) signals so as to estimate an ambient noise signal from the remaining signal after subtraction. A spectrum of the ambient noise signal is subtracted from a spectrum component of the input signals so as to reduce the ambient noise signal. Japanese Laid-open Patent Publication Hei 5-284588 discloses a sound (audio) signal input device having first and second microphones, in which an output signal of the second microphone is delayed and then phase-reversed. The thus phase-reversed output signal of the second microphone and the output signal of the first microphone are summed and amplified so as to cancel ambient noise. Further, Published Japanese Translation of PCT Application No. 2002-507334 discloses a noise control device having a curved reflector to deflect ambient noise so as to eliminate ambient noise. However, these known devices or systems do not solve the above problem.